Metal nanoparticles and nanowires are the subject of current research efforts motivated by their high potential utility derived from nanoscale induced optical, electrical and chemical properties.
A wide range of techniques has been reported to synthesize metal nanoparticles including numerous high vacuum approaches as well as a range of photochemical [1-3] and thermal methods [4-7]. A technique that is just beginning to gain attention is the potential use of zeolite surfaces to induce the growth of metal nanostructures [8-10]. With many of their properties manifested on a nano and subnano dimensional scale, molecular sieves would appear to be excellent candidates to be in the vanguard of such nanofabrication efforts [11].
Unfortunately, current techniques for nanosilver generation are expensive and cumbersome [14]. Subnanometer silver ensembles can be induced to form within zeolite cavities under certain conditions [15-18], and much larger configurations often form on zeolite surfaces under reductive atmospheres. While metals readily congregate on zeolite surfaces, achieving stable, zeolite supported metal nanoscale structures has proved difficult because of the high metal mobility generally seen on zeolite surfaces. Typically, upon reduction, metals ion-exchanged into zeolite crystals diffuse to the crystal surface and rapidly coalesce into micron-scale agglomerates [19, 20]. Because of the low surface to volume ratio of these agglomerates (compared to nanometal ensembles), they generally behave like bulk metals, not displaying the novel properties anticipated for nanoparticulates.
Nanoparticulate silver has many potential uses. Many useful properties might be expected if inexpensive nanostructured silver materials were readily available. Silver is a well-known antimicrobial agent and nanoscale silver is finding increasing usage in bandages and related medical applications [12, 13]. Nano-silver particulates are on the forefront of infection control in medical devices and bandages [13]. Current methods to generate nanosilver center on complex techniques such as surface sputtering. Research level work in biomedical engineering implants is showing promise in nanosilver bone cements where nanoparticle size control ranges from 5 nm to 50 nm [24]. Powerful surface plasmon absorption of nanoparticulate silver makes them particularly useful in applications such as biosensors, for example. Silver nanodots may be photo-fluorescence markers, which make them useful for a number of medical and similar applications. They are environmentally and biologically benign. Other exemplary silver nanodot applications include smart windows, rewritable electronic paper, electronic panel displays, memory components, and others.
A wide range of techniques has been reported to synthesize metal nanodots. Silver nanodots and their formation have recently been discussed by Metraux and Mirkin, 2005 [14]. Traditional methods for the production of silver nanodots require use of potentially harmful chemicals such as hydrazine, sodium borohydride and dimethyl formamide (“DMF”). These chemicals pose handling, storage, and transportation risks that add substantial cost and difficulty to the production of silver nanodots. A highly trained production workforce is required, along with costly production facilities outfitted for use with these potentially harmful chemicals.
Another disadvantage of known methods for producing silver nanodots relates to the time and heat required for their production. Known methods of production utilize generally slow kinetics, with the result that reactions take a long period of time. The length of time required may be shortened by some amount by applying heat, but this adds energy costs, equipment needs, and otherwise complicates the process. Known methods generally require reaction for 20 or more hours at elevated temperatures of 60° to 80° C., for example. The relatively slow kinetics of known reactions also results in an undesirably large particle size distribution and relatively low conversion. The multiple stages of production, long reaction times at elevated temperatures, relatively low conversion, and high particle size distribution of known methods make them costly and cumbersome, particularly when practiced on a commercial scale.
While silver ensembles are well known to form within zeolite cavities under certain conditions, and much larger configurations often form freely on zeolite surfaces, nanodots have not been known to form on zeolite surfaces.
These and other problems with presently known methods for making silver nanodots are exacerbated by the relatively unstable nature of the nanodots. Using presently known methods, silver nanodots produced have only a short shelf life since they tend to quickly agglomerate.
Therefore, there is a need in the art for a convenient and inexpensive method of forming metal nanodots, such as silver nanodots, which mitigates the difficulties of the prior art.